The present invention is directed to the layering of a liquid sample onto a support liquid, such as a gradient solution in a centrifuge tube and, more particularly, to the layering of a liquid sample through a narrow filler passage in an elongated neck protruding from the small access opening in a substantially closed centrifuge tube.
The typical centrifuge tube has a generally uniform cylindrical shape with one end closed and the other end being completely open to receive the fluid sample to be subjected to centrifugation. In a conventional open top tube, after the density gradient has been formed in the tube, it is relatively easy to layer the sample onto the top surface of the gradient by feeding it through a syringe against the inside of the tube just above the top of the gradient. Thus, the sample more or less spreads across the top surface of the gradient, essentially floating there until the centrifuge run begins. Generally, the gradient and sample are in aqueous solution or suspension and are, therefore, miscible. The lower the density difference, the more likely that the sample addition will stir and disturb the gradient. After the sample is introduced into the conventional tube, a tube cap is carefully attached across the open top and secured without disturbing the gradient or the sample and then the tube is placed in a suitable centrifuge for centrifugation.
In a recent development, Steven T. Nielsen developed a new type of centrifuge tube, which makes sealing of the tube and support thereof within a centrifuge much simpler and more reliable. A U.S. patent application Ser. No. 912,698 assigned to Beckman Instruments, Inc., the assignee of the present invention, has been filed concurrently with the present application on the substantially closed tube developed by Nielsen. The invention of the said Steven T. Nielsen was made prior to the present invention and nothing is herein claimed as my invention that is shown or described in the Nielsen application, which is to be regarded as prior art with respect to this application.
While the Nielsen centrifuge tube is substantially closed and may be sealed against the hydrostatic forces during centrifugation, it does not lend itself to the standard technique of layering a sample onto the surface of the supporting gradient. The Nielsen tube is a substantially closed cylindrically shaped tube having a small access or filler opening in one end thereof. Protruding from the opening is a neck or stem enclosing a filler passage through which liquid is inserted into the tube. In a preferred form, the Nielsen centrifuge tube is in the general shape of a "hot dog" with a narrow stem or neck protruding therefrom. The neck or stem is usually centered in the end of the tube and is elongated to the extent that it makes it extremely inconvenient, if not impossible, to layer the sample in the usual manner onto a supporting gradient. Because the tube material is more or less hydrophobic, it is impractical to feed the sample against the inside of the tube neck with the expectation that it will flow by surface tension down the neck and across the underside of the tube shoulder and spread smoothly across the top of the gradient.